1. Field of the Invention
The present invention relates to a magnetoresistive effect film which is used in a magnetoresistive effect element which reads the magnetic field strength from a magnetic recording medium or the like as a signal, and more specifically to magnetoresistive effect film that has a large resistance change ratio in a small external magnetic field.
2. Description of Related Art
In recent years, transducers known as MR (magnetoresistive) sensors (or MR heads) have been developed which read data from a magnetic surface with a high linear density.
By means of a reading element, an MR sensor performs detection of a magnetic field signal via a change of resistance, which is a function of the magnetic flux strength and direction.
In an MR sensor Such as this in the past, one component of the resistance of the reading element changed in proportion to the square of the cosine of the angle formed between the direction of the magnetization and the detection current flowing in the element, according to an operating principle known as the anisotropic magnetoresistance (AMR) effect. The AMR effect is discussed in detail in D. A. Thompson et al "Memory, Storage, and Related Applications" IEEE Transactions of Magnetics. MAG-11, p. 1039 (1975).
Additionally, there has recently been a report of a more prominent magnetoresistive effect, whereby the resistance change in a laminated magnetic sensor is attributed to spin dependency transmission between magnetic layers with an intervening non-magnetic layer, and to accompanying spin dependency dispersion at the layer boundary.
This magnetoresistive effect is known by a variety of names, including "giant magnetoresistive effect" and "spin valve effect".
A magnetoresistive sensor such as this is made from an appropriate material, and provides an improvement in density and increase in resistance change when compared to observation by a sensor which uses the AMR effect. With this type of MR sensor, the internal planar resistance between a pair of antiferromagnetic layers that are separated by a non-magnetic layer varies in proportion to the cosine of the angle formed between the magnetization directions in the two layers.
In the Japanese Unexamined Patent Publication No. 2-61572, there is description of a laminated magnetic structure which brings about a large MR effect, that occurs by means of anti-parallel orientation of the magnetization between the magnetic layers.
In the above-noted Japanese unexamined patent application publication, transition metals and alloys thereof are cited as materials that can be used in the magnetic layers in this laminated structure. There is disclosure that FeMn is suitable for use as at least one of two magnetic layers that are separated by a center layer.
In the Japanese Unexamined Patent Publication No. 4-358310, there is a disclosure of an MR sensor having a two layers of thin film ferromagnetic material which are separated by a non-magnetic metallic thin film, in which when the applied magnetic field is zero the magnetization directions in the two ferromagnetic thin films are mutually perpendicular, the resistance between the two non-coupled ferromagnetic layers varying in proportion to the cosine of the angle formed between the magnetization directions in the two layers, this being independent of direction of current flow in the sensor.
In the Japanese Unexamined Patent Publication No. 6-214837, there is a disclosure of magnetoresistive effect element in which, onto a substrate a plurality of magnetic thin films are laminated via an intervening non-magnetic layer, wherein one soft magnetic thin film which adjacently arranged to each other, via an intervening non-magnetic thin film, is provided with an antiferromagnetic thin film, and further wherein in the magnetoresistive effect film in which the bias magnetic field on this antiferromagnetic thin film is Hr and the coercivity of the soft magnetic thin film is Hc2 (&lt;Hr), the above-noted antiferromagnetic thin film is a super lattice selected as at least two types from the group consisting of NiO, NixCo1-x 0, and CoO.
Additionally, in the Japanese Unexamined Patent Publication No. 7-136670 in a magnetoresistive effect film having the same structure as in the Japanese Unexamined Patent Publication No. 6-214837, there is a disclosure of a magnetoresistive effect element that is a two-layer film wherein onto a antiferromagnetic thin film of NiO, is laminated a layer of CoO to a thickness of 10 to 40 .ANG..
However, in a magnetoresistive effect element in the past such as noted above, the following problems existed.
(1) Although operation is by means of a small external magnetic field, a practically usable sensor or magnetic head must have a signal magnetic field applied in the direction of its easy axis, this leading to the problems that, for use as a sensor, there is no change in resistance exhibited in the area of a zero magnetic field, and that there is a non-linearity occurring due to effects such as the Barkhausen jump.
(2) There is ferromagnetic Interaction between a magnetic layers which neighbor one another via an intervening non-magnetic layer, causing the problem of a shift of linear region of the MR curve away from the zero field.
(3) It is necessary to use a material such as FeMn, which is subject to corrosion, as the antiferromagnetic thin film, making it necessary to take measures in a practically usable device such as using an additive or applying a protective film.
(4) In the case in which a nickel oxide film, which has good resistance to corrosion, is used as the antiferromagnetic thin film, the bias magnetic field is small, and the coercivity of a neighboring soft magnetic film becomes large, leading to the difficulty in achieving magnetization antiparallelness between magnetic layers.
(5) In the case of using a nickel oxide film, the blocking temperature (at which the bias magnetic field is lost) is low, so that if heat treatment is done at 250.degree. C. or above, the bias magnetic field is reduced.
(6) In the case in which an oxide antiferromagnetic film is used as the antiferromagnetic film, oxidation of adjacent soft magnetic films occurs when heat treatment is done, resulting in a reduction of the resistance change ratio in the magnetoresistive effect film.
(7) Because the structure basically obtains a change in resistance by using the change in the mean free path of conducting electrons in a three-layer structure of a magnetic thin film, a non-magnetic thin film and another magnetic thin film, compared with a magnetoresistive effect film known as a coupling type, which has a multiple layer structure, the resistance change ratio is small.
An object of the present invention is to provide a magnetoresistive effect film which exhibits a large linear resistance change in the region of zero magnetic field, and which has good immunity to both corrosion and heat.
A magnetoresistive effect film according to the present invention minimally has a lamination of an antiferromagnetic thin film, a first magnetic thin film (soft magnetic thin film) which makes contact with the above-noted antiferromagnetic thin film, a non-magnetic thin film which makes contact with the first magnetic thin film, and a second magnetic thin film (soft magnetic thin film) which makes contact with the above-noted non-magnetic thin film, wherein if the bias magnetic field of the above-noted antiferromagnetic thin film is Hr and the coercivity of the above-noted second magnetic thin film is Hc2, the equation of Hc2 &lt;Hr should be fulfilled:
The antiferromagnetic thin film, the first magnetic thin film, the non-magnetic thin film, and the second magnetic thin film can be laminated onto a substrate in this sequence, starting with the antiferromagnetic thin film, and an also be laminated onto a substrate in the reverse sequence, starting with the second magnetic thin film. The above-noted antiferromagnetic thin film is a laminate of a nickel oxide film and an iron oxide film having a thickness in the range 20 to 100 .ANG..
By virtue of the above structure, in comparison with the structure used in the past, there is a prominent improvement in unidirectional anisotropy in the exchange coupling film, and a prominent improvement in thermal stability, thereby enabling the achievement of stable operation of the magnetoresistive effect element.
It is desirable that the thickness of the nickel oxide film be 1000 .ANG. or less. While a greater thickness does not result in a deterioration of the effect, from the standpoint of reading accuracy in the magnetoresistive effect element, this is desirable because of a reduction of accuracy that accompanies an increase in the film thickness.
Because the lower limit of the thickness of the nickel oxide film is greatly influenced by the size of the exchange coupling magnetic field applied to magnetic thin films having neighboring crystalline structures in the antiferromagnetic thin film, it is preferable that this be made 100 .ANG. or greater, at which there is a good crystallization. By forming films onto the substrate by heating it from room temperature to 300.degree. C., crystallization is improved and the bias magnetic field increases.
Films are grown by such methods as vapor deposition, sputtering, molecular beam epitaxy (MBE) and the like. AS the substrate, it is possible to use materials such as quartz glass, Si, MgO, Al2O3, GaAs, ferrite, CaTi2O3, BaTi2O3, and Al2O3-TiC.
In the present invention, by inserting a metal of a thickness of 3 to 30 .ANG., selected as one or more metals from the group consisting of nickel, iron, and cobalt, between the antiferromagnetic thin film and the magnetic thin film, oxidation of the magnetic thin film is suppressed, and there is also a significant improvement in the reduction in the exchange coupling magnetic field when heat treating and in a reduction of the resistance change ratio.
In the present invention, by making the surface roughness of the antiferromagnetic thin film 2 to 15 .ANG., there is a change in the magnetic domain structure in the magnetic thin film laminated thereover, this causing a reduction in the coercivity of the exchange coupling film.
It is preferable that the material used for the magnetic thin film in the present invention be one or more metals selected from a group consisting of Ni, Fe, Co, FeCo, NiFe, and NiFeCo. By doing this, a large dispersion of conduction electrons occurs in the boundary between the non-magnetic thin film and the magnetic thin film, thereby achieving a large resistance change.
In the present invention, a magnetic thin film is formed from above-noted magnetic materials. In particular, the implementation of the present invention is made possible by selecting a material for which the anisotropic magnetic field Hk2 of the magnetic film which is not adjacent to the antiferromagnetic thin film is greater than the coercivity Hc2.
The anisotropic magnetic field can also be made large by making the film thickness small. For example, if a NiFe film is made to a thickness of approximately 10 .ANG., it is possible to make the anisotropic magnetic field Hk2 larger than the coercivity Hc2.
Additionally, it is possible to manufacture the above-noted magnetoresistive effect element so that the easy magnetization axis of the magnetic thin film is perpendicular with respect to the direction of the applied signal magnetic field, and so that the coercivity of the magnetic thin film in the direction of the signal magnetic field is such that Hc2&lt;Hk2&lt;Hr, by forming the magnetic thin film within a magnetic field.
Specifically, the applied magnetic field is rotated by 90 degrees during the formation of film, so that the easy axis of the magnetic thin film which is adjacent to the antiferromagnetic thin film is perpendicular to the easy magnetization direction of magnetic thin film that is neighboring thereto via an intervening non-magnetic film, or alternately to rotate the substrate by 90 degrees in the magnetic field.
It is desirable that the thickness of each magnetic thin film be 150 .ANG. or less. If the film thickness is made larger than 150 .ANG., accompanying the increase in film thickness there is an increase in the region that does not contribute to electron dispersion, resulting in a reduction in the giant magnetoresistive effect.
While there is no particular lower limit to the thickness of the magnetic thin films, at a thickness below 10 .ANG., there is a large surface dispersion of conduction electrons, resulting in a reduction in the magnetoresistive change.
If the thickness is made 10 .ANG. or greater, it is easy to maintain a uniform film thickness, and good characteristics are achieved. Additionally, there is no problem of the saturation magnetization becoming small.
The coercivity of a magnetic thin film which is adjacent to an antiferromagnetic thin film can be made small by raising the substrate temperature from room temperature to 300.degree. C. and continuously forming it continuously with the antiferromagnetic thin film.
In addition, by inserting Co, FeCo, NiCo, or NiFeCo at the boundary between the magnetic thin film and the non-magnetic thin film, the probability of conduction electron boundary dispersion is increased, thereby enabling achievement of a large resistance change,
The lower limit of the inserted thin film is 3 .ANG.. At below this, there is not only a reduced effect of insertion, but also it becomes difficult to control the growth of the film.
While there is no particular upper limit on the thickness of the inserted film, it is desirable that this be approximately 40 .ANG..
At greater than this, hysteresis occurs in the output in the operation range of the magnetoresistive effect element.
Additionally, in a magnetoresistive effect element such as this, by bringing a permanent magnet thin film into proximity in the easy magnetization direction of the magnetic thin film which detects the external magnetic field, that is, the magnetic field which is not adjacent to the antiferromagnetic thin film, it is possible to achieve stabilization of magnetic domains, and to avoid non-linear output such as caused by a Barkhausen jump. It is preferable to use a material such as CoCr, CoCrTa, CoCrTaPt, CoCrPt, CoNiPt, CoNiCr, CoCrPtSi, or FeCoCr as the material for the permanent magnet thin film. It is also possible to use Cr or the like as a base layer for these permanent magnet thin films.
The non-magnetic thin film is a material which serves the purpose of weakening the magnetic coupling between magnetic thin films and, to achieve high resistance change and immunity to heat, it is desirable that this be made of one or more metals from the group consisting of Cu, Au, Ag, and Ru.
From experiments, it is desirable that the thickness of the non-magnetic thin film be in the range from 20 to 35 .ANG..
In general, if the film thickness exceeds 40 .ANG., the resistance is established by the non-magnetic thin film, the dispersion effect which is dependent upon spin becoming relatively small, this resulting in a small magnetic resistance change ratio.
If the film thickness is greater than 20 .ANG., the magnetic interaction between magnetic thin films becomes excessively great, and it becomes impossible to avoid the occurrence of the direct magnetic contact condition (pinholes), resulting in a condition in which the magnetization directions of the magnetic thin films are different.
In a spin valve film which uses an oxide antiferromagnetic thin film such as in the present invention, because there is a mutual interaction between magnetic thin films with respect to the thickness of the non-magnetic thin film, with the thickness of the non-magnetic thin film in the region from 8 to 12 .ANG., the two magnetic thin films are antiferromagnetically coupled, resulting in a large resistance change in the region near zero magnetic field.
The thickness of the magnetic thin film or the non-magnetic thin film can be measured, for example, by a transmission-type electron microscope, a scanning-type electron microscope, or by Auger electron spectroscopy.
The crystallization structure of the thin film can be verified by such means as X-ray diffraction or high-speed electron beam diffraction.
When configuring a magnetoresistive effect element, there is no particular restriction to the number of lamination repetitions N of an artificial matrix film, it being possible to establish this in accordance with the desired magnetic resistance change ratio.
However, because the antiferromagnetic thin film resistivity is large, resulting in loss of lamination effect, it is preferable that this structure be replaced by a antiferromagnetic layer/magnetic layer/non-magnetic layer/magnetic layer/non-magnetic layer/magnetic layer/antiferromagnetic layer structure.
The surface of the uppermost magnetic thin film does not need to be provided with an oxidation preventing film of silicon nitride, silicon oxide, alumina or the like, and it is possible to provide a metallic conductive layer for the purpose of Making wiring connections to electrodes.
Because it is not possible to directly measure the magnetic characteristics of the magnetic thin films that exist within the magnetoresistive effect element, the measurement is usually made as follows.
The magnetic thin film to be measured is formed in a measurement sample, with alternate films of this film and a non-magnetic thin film grown so as to reach a total thickness of 500 to 1000 .ANG., the magnetic characteristics of this laminate being then measured. In this case, the thickness of the magnetic thin film, the thickness of the non-magnetic thin film, and the composition of the non-magnetic thin film are the same as used in the magnetoresistive effect element.
Using a magnetoresistive effect element according to the present invention, an antiferromagnetic thin film is formed so as to be adjacent to the first magnetic thin film, so that an exchange bias inevitably operates.
The reason for this is that the principle of the present invention is based on the fact that when the directions of magnetization of magnetic thin films which are adjacent via an intervening non-magnetic thin film are mutually opposite, the maximum resistance is exhibited.
That is, according to the present invention, as shown in FIG. 3, when the external magnetic field H has a magnitude that falls between the anisotropic magnetic field Hk2 of the second magnetic thin film and the anti-magnetic force, i.e., bias magnetic field, Hr of the first magnetic thin film, that is when the condition Hk2&lt;H&lt;Hr is satisfied, the directions of magnetization in adjacent magnetic thin films are mutually opposite, resulting in an increase in resistance.
FIG. 2 is an exploded perspective view which shows an example of an MR sensor which uses an magnetoresistive effect element according to the present invention.
This MR sensor, as shown in FIG. 2, is formed from an artificial lattice films 7 that is formed on the substrate 4, in which on top of an antiferromagnetic thin film 5 that is formed on the substrate 4, there are formed magnetic thin films 3 and 2, with an intervening non-magnetic thin film 1, this magnetic thin films 2 and 3 having mutually perpendicular easy magnetization directions, a signal magnetic field which is released from the magnetic recording medium 8 being established so as to be perpendicular to the easy magnetization direction of the magnetic thin film 2.
In this arrangement, the magnetic thin film 3 has imparted to it unidirectional anisotropy by the antiferromagnetic thin film 5 which is adjacent thereto.
Because the magnetization direction of the magnetic thin film 2 rotates in response to the magnitude of the signal magnetic field from the magnetic recording medium 8, the resistance changes and the magnetic field is thereby sensed.
Next, the relationship between the external magnetic field, the coercivity, and the magnetization direction will be described. As shown in FIG. 3, the anti-magnetic force, i.e., bias magnetic field, or the magnetic thin film 3, which is exchange biased, is Hr, the coercivity of the magnetic thin film 2 is Hc2, and the anisotropic magnetic field is Hk2 (where 0&lt;Hk2&lt;Hr).
At first, the external magnetic field B is applied so that H&lt;-Hk2, (region (A)). Under this condition, the magnetization directions of the magnetic thin films 2 and 3 are the same as H (negative direction).
Next, as the external magnetic field is weakened, under the condition--Hk2&lt;H&lt;Hk2 (region (B)), the magnetization of the magnetic thin film 2 rotates so as to reverse to the positive direction, and when Hk2&lt;H&lt;Hr (in region (C)), the magnetization directions of the magnetic thin films 2 and 3 are mutually opposite.
When the external Magnetic field H is further increased so that Hr&lt;H (region (D)), the magnetization direction of the magnetic thin film 3 also reverses, so that the magnetization directions of both magnetic thin film 2 and magnetic thin film 3 are positive.
As shown in FIG. 4, the resistance of this film varies in response to the relative magnetization directions of magnetic thin films 2 and 3, this varying linearly in the region of zero magnetic field, the maximum value Rmax being exhibited in region (C).